Broadband wireless systems are in a rapidly evolutionary phase in terms of the development of various technologies, development of various applications, deployment of various services and generation of many important standards in the field. Although there are many factors to be considered in the design of these systems, the key factors have been the bandwidth utilization efficiency due to the limited bandwidth allocation, flexibility in operation and robustness of the communication link in the presence of various disturbances while achieving the specified performance. At present, the OFDM and spread spectrum techniques have been adapted in many wireless communication standards, such as the World-wide Interoperability for Microwave ACCESS (Wimax), digital audio broadcasting (DAB), digital video broadcasting-terrestrial (DVB-T), Long Term Evolution (LTE), Universal Mobile Telecommunications Service (UMTS) or Code Division Multiple Accessing (CDMA) 2000, Wideband CDMA (W-CDMA), CDMA standard IS95, IEEE 802.11 Wireless Local Area Network (WLAN), IEEE 802.15 Wireless Private Area Network (WPAN), etc.
One of the advantages of the OFDM system is the mitigation of a major source of distortion present in high data rate wireless communication links, namely the inter symbol interference (ISI) achieved by increasing the symbol period by the use of multiple carrier transmission. However, the use of a large number of carriers based on the orthogonality property in the OFDM system makes the performance of the system very sensitive to any carrier frequency offsets introduced, for example, by the Doppler shifts encountered in the wireless channels. The proper operation of the OFDM system requires means for precise estimate of the Doppler that may be different for different carriers in the frequency selective fading channel, and means to mitigate such a Doppler effect from the received OFDM signal. Another important problem arising with the use of a relatively large number N of carriers used in the OFDM signal is a relatively high peak to average power ratio resulting in a much reduced radio frequency (RF) power amplifier efficiency. Various methods exist in the prior art to solve these problem.
In multiple accessing mode of the OFDM system wherein relatively narrow band OFDM channels are assigned to various users in a mobile communication network, the frequency selective fading of the communication channel may cause severe fading of some of the user signals resulting in frequent hand offs or call drop for such users in a mobile communication network. The solution to such a problem may be the transmission of the user signal over multiple OFDM channels for providing a diversity gain and/or transmission at a much reduced data rate. However, such a solution results in a reduced capacity of the OFDM system. Another problem with the OFDM system is that the narrow band signals of various users have no protection against any intended or unintended interference. Any significant narrowband interference in any of the OFDM channels may disrupt communication to users assigned such channels.
Spread spectrum systems provide protection against narrow band interference and possess various other desirable properties such as graceful degradation with increased number of active multiple access users, etc. There are mainly two types of spread spectrum systems in use in the mobile communication networks, viz., the direct sequence spread spectrum (DSSS) system and the frequency hopped spread spectrum (FHSS) system. In the DSSS system, the baseband modulated signal is modulated in a second stage by a pseudo random binary sequence resulting in a spreading of the signal bandwidth that results in the protection against narrow band interference. However, in the wireless networks, due to the increased self noise in the system resulting from multipath propagation, the DSSS system may be more suited for a relatively short range multiple accessing application or long range point to point communication by using separation of the multipath components using, or example, a rake receiver. In the FHSS system, the carrier frequency of the baseband modulated signal is varied according to some pseudo random (PN) pattern or sequence that varies at the information symbol rate or a integer multiple thereof in the fast FHSS system, and results in the spreading of the signal bandwidth that depends upon the number of frequencies in the pseudo random pattern. The FHSS system may offer more protection against multipath over relatively longer distances compared to the DSSS system.
In the FHSS system, different users are assigned different pseudo random frequency sequences. A disadvantage of the FHSS system may be that whenever, the frequencies of more than one PN sequence coincide, that results in a collision whereby the symbols during such a collision period are erased placing a floor on the probability of symbol error irrespective of the signal to noise ratio or other parameters. Similarly the presence of narrow band interference around any one of the hop frequencies may result in the erasure of the symbol being transmitted during that hop. For example, if 2 out of 100 hop frequencies are interfered with, the probability of symbol error will be lower bonded by 0.02. This is different compared to the DSSS system wherein the narrow band interference is spread out in a wide band in the despreader at the receiver causing a relatively small degradation in the probability of symbol error. Moreover, the FHSS system requires a frequency synthesizer for the generation of pseudo random frequency sequence capable of generating a relatively large number of frequencies with relatively high switching speeds equal to the information symbol rate, or a integer multiple thereof with relatively low noise and a relatively high precision frequencies present in the pseudo random frequency sequence.
Kumar teaches an orthogonal frequency chirp multiple accessing (OFCM) spread spectrum system in Orthogonal Frequency Chirp Multiple Accessing Systems and Methods, U.S. patent application Ser. No., 14/244,774, 2014, wherein the baseband modulated signals are modulated by a time varying frequency waveforms such that an orthogonality is maintained among the various multiple accessing signals. The OFCM system taught by Kumar is a spectrally efficient and provides protection against both the interference and frequency selective fading due to multipath propagation. The OFCM system, however, may not provide any privacy protection to the multiple accessing users. As the time varying frequency waveforms are delayed versions of a common waveform whose frequency varies linearly with time, an unauthorized user can easily receive the baseband information symbols intended for the other multiple accessing users.
There is a strong motivation to come up with systems and methods that achieve the various advantages of the prior art spread spectrum systems while overcoming various possible weaknesses therein. The frequency hopped frequency modulation spread spectrum (FHFMSS) system of the invention provides protection against deep fades in some segments of the spectrum and against narrowband interference similar to that of the DSSS system in that the impact of any narrow band interference is distributed throughout the spread bandwidth after the despreading at the receiver rather than being concentrated in a few symbols as in the FHSS system of prior art placing a floor on the probability of symbol error. In the FHFMSS system, the wideband Frequency Hopped Frequency Modulation (FHFM) spreading waveform is generated by Frequency Modulation (FM) modulation of a multiplicity of the periodic waveforms with time varying frequencies determined by pseudo random sequences. The FHFM waveform in turn modulates the baseband modulated signal. A deterministic short sequence of frequency pairs has been previously used in the construction of a command destruct code in the space lift range system.
Unlike the FHSS system of the prior art, the spectrum of the FHFMSS signal occupies a wide bandwidth during any symbol period. Deep fading in a few segments of the wide band spectrum may not result in any significant performance degradation to any of the users. In the prior art FHSS system, the number of hop frequencies and the period of the pseudo random (PN) sequences may be both equal to the bandwidth spreading factor that is the ratio of the FHSS signal bandwidth and the baseband modulated signal bandwidth with the switching speed of the frequency synthesizer equal to or an integer multiple of the baseband information symbol rate. In the FHFMSS system of the invention, the various parameters such as the number of hop frequencies, the period of the PN code, the bandwidth spreading factor , and the switching speed of the frequency synthesizer may be selected independently so as to minimize the transmitter complexity while achieving the desired spread spectrum signal bandwidth. The FHFMSS has the advantage of not requiring a relatively very large number of hop frequencies and high switching speeds of the frequency synthesizer in the FHSS system of the prior art.
The FHFMSS system of the invention also inherits various advantages of the prior FHSS system in that it provides robustness against multipath propagation interference and even a better privacy protection compared to both the DSSS and FHSS system. In the FHFMSS system of the invention, individual user signals with modulations such as MPSK (Multi Frequency Shift Keying), are constant envelope signals inheriting the property from the frequency modulated (FM) signals with an advantage in terms of requiring relatively low amplifier back off in the user to base station transmission in a mobile communication network. Unlike the prior art DS spread spectrum systems, the power spectral density of the FHFMSS system of the invention does not have any spectral side lobes. These and other advantages of the FHFMSS system will be evident from the following specifications.